1. Field of the Invention
The present invention relates to a solid-state imaging device and manufacturing method thereof, wherein the solid-state imaging device is provided with a plurality of photoelectric conversion portions so arranged that they form a matrix, i.e., two-dimensional pattern or array constructed of their rows and their columns, and more particularly to a solid-state imaging device and manufacturing method thereof, wherein the solid-state imaging device is adapted for use in image sensors of various types of image input instruments, for example such as facsimiles, video cameras, digital still cameras and like instruments.
2. Description of the Related Art
Solid-state imaging devices have long been constructed of charge coupled devices (i.e., CCDs), and provided with a plurality of photoelectric conversion portions, wherein each of the photoelectric conversion portions converts incident light into signal charge the amount of which charge corresponds to the amount of the incident light, and the photoelectric conversion portions are so arranged that they form a matrix, i.e., two-dimensional pattern or array constructed of their rows and their columns. Of these solid-state imaging devices, particularly, one having a construction in which the photoelectric conversion portion and the charge transfer portion for transferring the signal charge are separately formed is capable of performing separately each of the process of photoelectric conversion, process of charge readout and the process of charge transfer, and, therefore capable of being driven in various drive modes. Due to this, the solid-state imaging device is characterized by its very wide range of applications.
This type of the solid-state imaging device is known, for example, in a Japanese magazine: xe2x80x9cEizo Jyohoxe2x80x9d, August issue for 1995, vol. 27, pp. 80-86. This solid-state imaging device is characterized in that: it doubles as a charge readout electrode for controlling the writing and the reading of the signal charge from the photoelectric conversion portion to the corresponding one of the charge transfer portions; and, the photoelectric conversion portion is formed by mask-alignment of the charge transfer electrode for controlling in transfer the signal charge of the corresponding one of the charge transfer portions.
Hereinbelow, a manufacturing method of the conventional solid-state imaging device disclosed in the above Japanese magazine (hereinafter referred to as the first conventional example) will be described in due order of manufacturing, i.e., process steps thereof with reference to FIGS. 33(a) to 37(b) First, as shown in FIG. 33(b), a p-type well layer 2 is formed by ion-implanting a p-type impurity such as boron ions B+ and like ions in an n-type semiconductor substrate 1. Then, formed in a surface region of the p-type well layer 2 by ion-implanting a p-type impurity such as boron ions B+ and like ions and an n-type impurity such as phosphorus ions P+ and like ions are: a P+ type channel stop 3 for isolating the devices from each other; a p-type charge readout portion 4 for retrieving the signal charge from the photoelectric conversion portion 6 (shown in FIG. 36(b)) to an n-type charge transfer portion 5; and, the n-type charge transfer portion 5 for transferring the signal charge thus retrieved. After that, as shown in FIG. 34(b), a photoresist film 7-is formed on the surface of the p-type well layer 2 other than an area in which the photoelectric conversion portion 6 will be formed later. Then, as shown in FIGS. 34(a) and 34(b), an n-type well 8 which will form the photoelectric conversion portion 6 later is formed by ion-implanting an n-type impurity such as phosphorus ions P+ and like ions at an acceleration energy of more than or equal to 200 KeV using the photoresist film 7 as a mask. Subsequent to this, the photoresist film 7 is removed. Then, a gate insulation film 9 constructed of a thermal oxidation film, oxidation film, nitride film, oxide (ONO) film, or like film is formed over the entire surface of the substrate. Then, a gate electrode film (not shown) such as a polysilicon film and the like is formed over the gate insulation film 9. Then, by removing an unnecessary region of the gate electrode film through a plasma etching process, a charge transfer electrode 10 is formed. Further formed on this charge transfer electrode 10 by using the thermal oxidation film and by a (CVD) chemical vapor deposition process is a CVD oxidation film which forms an interlayer insulation film (not shown). After forming the interlayer insulation film, as shown in FIGS. 35(a) and 35(b), a charge transfer electrode 11 which doubles as a charge readout electrode is formed over both the gate insulation film 9 and the interlayer insulation film.
As shown in FIGS. 36(a) and 36(b), the photoelectric conversion portion 6 is formed by self-alignment using the charge transfer electrodes 10 and 11 as masks in an ion implantation process of the p-type impurity such as boron ions B+ and like ions in a shallow surface region of the n-type well layer 8, and thereby forming a P+ type region 12 for preventing dark current from occurring, which dark current occurs in the surface of the photoelectric conversion portion 6 to impair the SN (Signal-to-Noise) ratio at a time when the intensity of illumination is low. At this time, in order to prevent the above p-type impurity from being ion-implanted in the other region where charge detection portions and on-chip amplifiers and the like are formed, it is necessary to form a photoresist film over the other region described above. Then, an interlayer insulation film 13 is formed over the entire surface of the substrate. After that, as shown in FIGS. 37(a) and 37(b), a light shield film 14 made of tungsten, aluminum and like materials is formed over the interlayer insulation film 13 to prevent the same 13 from being exposed to light. Then, the light shield film 14 thus formed over the photoelectric conversion portion 6 is removed to form an opening portion 14a. Each of the n-type well layer 8 of the photoelectric conversion portion 6 and the p-type well layer 2 formed thereunder functions as a buried-type photodiode.
In the conventional solid-state imaging device produced by the manufacturing method described above, as shown in FIG. 35(b), since the photoelectric conversion portion 6 is formed by mask-alignment using the edge portion 11a of the charge transfer electrode 11 as a mask, the conventional solid-state imaging device suffers from problems of large variations in readout voltage of the signal charge, which variations are caused by misalignment. Further, when a gap is produced between the photoelectric conversion portion 6 and the edge portion 11a of the charge transfer electrode 11, the readout voltage remarkably increases. Consequently, in order to prevent the readout voltage from remarkably increasing, it is necessary to protrude the edge portion 11a by a distance of at least the corresponding amount of misalignment occurring in mask alignment, so that the edge portion 11a overlies the photoelectric conversion portion over the above distance. This reduces the opening portion 14a in area size, as shown in FIG. 37(b). The opening portion 14a thus reduced in area size increases the tendency of the incident light to be reflected from the light shield film 14.
In order to solve the above problem, Japanese Patent Laid-Open No. Hei5-6992 discloses another conventional solid-state imaging device in which a photoelectric conversion portion is formed by self-alignment using the edge portion of a charge transfer electrode as a mask.
Hereinbelow, a manufacturing method of the another conventional solid-state imaging device disclosed in the above document (hereinafter referred to as the second conventional example) will be described in due order of manufacturing, i.e., process steps thereof with reference to FIGS. 38(a) and 42(b).
First, as shown in FIGS. 38(a) and 38(b), a p-type well layer 22 is formed by ion-implanting a p-type impurity such as boron ions B+ and like ions in an n-type semiconductor substrate 21. Then, formed in a surface region of the p-type well layer 22 by ion-implanting a p-type impurity such as boron ions B+ and like ions and an n-type impurity such as phosphorus ions P+ and like ions are: a P+ type channel stop 23; a p-type charge readout portion 24; and, an n-type charge transfer portion 25. Then, a gate insulation film 26 constructed of a thermal oxidation film, oxidation film, nitride film, oxide (ONO) film, or like film is formed over the entire surface of the substrate. Subsequent to the above, a gate electrode film (not shown) such as a polysilicon film and the like is formed over the gate insulation film 26. Then, by removing an unnecessary region of such gate electrode film by a plasma etching process, a charge transfer electrode 27 is formed. Further formed on this charge transfer electrode 27 is an interlayer insulation film which is constructed of the thermal oxidation film, CVD oxidation film and like films. After forming the interlayer insulation film, as shown in FIGS. 39(a) and 39(b), a gate electrode film 28 constructed of a polysilicon film and the like is formed over both the gate insulation film 26 and the interlayer insulation film. Then, formed on these films is a photoresist film 31 which is provided with an opening portion. This opening portion covers a region other than a part 30 thereof, in which region the photoelectric conversion portion 29 (described later) will be formed later. After that, by removing an unnecessary region of the gate electrode film 28 through a plasma etching process, a pattern which will form a charge transfer electrode 32 is formed, wherein the charge transfer electrode 32 doubles as a charge readout electrode. Then, the photoresist film 31 is removed. Subsequent to the above, formed again on both the gate insulation film 26 and the above interlayer insulation film is a photoresist film 33. This film 33 is provided with an opening portion which covers the above part 30 of the region in which the photoelectric conversion portion 29 will be formed later. After that, the above part 30 is removed by the plasma etching process using the photoresist film 33 as a mask, so that a charge transfer electrode 32 is formed. Subsequent to the above, as shown in FIGS. 40(a) and 40(b), an n-type well 34 forming the photoelectric conversion portion 29 is self-aligned with an edge portion 32a of the charge transfer electrode 32, and formed by ion-implanting an n-type impurity such as phosphorus ions P+ and like ions at an acceleration energy of more than or equal to 200 KeV using both the charge transfer electrode 32 and the photoresist film 33 as masks.
Then, the photoresist film 33 is removed. After that, the photoelectric conversion portion 29 is formed by self alignment using both the charge transfer electrodes 27 and 32 as masks in an ion implantation process of the p-type impurity such as boron ions B+ and like ions in a shallow surface region of the n-type well layer 34, and thereby forming a P+ type region 35 for preventing dark current from occurring, which dark current occurs in the surface of the photoelectric conversion portion 29 to impair the SN (Signal-to-Noise) ratio at a time when the intensity of illumination is low. At this time, in order to prevent the above p-type impurity from being ion-implanted in the other region where charge detection portions and on-chip amplifiers and like components of the solid-state imaging device are formed, it is necessary to form a photoresist film over the other region described above. Then, as shown in FIG. 42(b), an interlayer insulation film 36 is formed over the entire surface of the substrate. After that, as shown in FIGS. 42(a) and 42(b), a light shield film 37 made of tungsten, aluminum and like materials is formed over the interlayer insulation film 36 to prevent the same 36 from being exposed to light. Then, the light shield film 37 thus formed over the photoelectric conversion portion 29 is removed to form an opening portion 37a. The n-type well layer 34 of the photoelectric conversion portion 29 and the p-type well layer 22 formed thereunder function as a buried-type photodiode.
In the conventional solid-state imaging device produced by the manufacturing method described above, as shown in FIG. 42(b), the photoelectric conversion portion 29 is formed by self alignment using the edge portion 32a of the charge transfer electrode 32 as a mask, wherein the charge transfer electrode 32 doubles as a charge readout electrode. Consequently, it is possible for this second conventional example of the solid-state imaging device to prevent the readout voltage from varying due to the misalignment problem inherent in the first conventional example of the solid-state imaging device. Further, in the second example of the conventional solid-state imaging device, since the photoelectric conversion portion 29 is aligned with the edge portion 32a without fail, there is no gap between the photoelectric conversion portion 29 and the edge portion 32a, which prevents the readout voltage from being remarkably increased. As a result, in contrast with the first example of the conventional solid-state imaging device, in this second example of the conventional solid-stage imaging device, the edge portion 32a is not required to protrude in a manner such that the edge portion 32a overlies the photoelectric conversion portion 29. This permits the opening portion 37a to be larger in area size than the corresponding opening portion of the first example of the conventional solid-state imaging device, and, therefore decreases the tendency of the incident light to be reflected from the light shield film 37.
However, the second example of the conventional solid-state imaging device suffers from the following problem.
Namely, in forming the charge transfer electrode 32, the pattern in which the charge transfer electrode 32 is formed and a part of the gate electrode film 28 corresponding to the part 30 of the photoelectric conversion portion 29 is included is formed as shown in FIGS. 43(a) and 43(b). After that, as shown in FIGS. 44(a) and 44(b), a second etching process of the plasma etching type is carried out over the entire region of the photoelectric conversion portion 29 including the part 30. In this second etching process, in addition to the part 30 on which the gate electrode film 28 still remains, another part or region of the photoelectric conversion portion 29 on which only the gate insulation film 26 remains is also simultaneously subjected to the second etching process. Due to such simultaneous etching, as indicated by the reference numeral 38 in FIG. 44(b), a boundary area between the photoelectric conversion portion 29 and the gate insulation film 26 has a tendency to be damaged.
The following is an example in which: an oxide film having a film thickness of 800 angstroms is formed to serve as the gate insulation film 26; and, a polysilicon film having a film thickness of 3000 angstroms is formed to serve as the charge transfer electrode 32. In the first etching process, in order to form the pattern which will form the charge transfer electrode 32 without producing any residues in a side wall portion of the charge transfer electrode 32 in the etching process, it is necessary to etch away both the charge transfer electrode 32 and the gate insulation film 26 up to a depth of 6000 angstroms corresponding to two times the thickness of the actual polysilicon film. In this case, an excessively etched-away depth of the polysilicon film reaches (6000xe2x88x923000=3000) angstroms. Consequently, when the selective ratio of the polysilicon film to the oxide film is equal to a ratio of 10 to 1, the gate insulation film 26 is etched away up to a depth of 300 angstroms. Then, in the second etching process, when the gate electrode film 28 corresponding to the part 30 of the photoelectric conversion portion 29 is removed, in order to carry out a complete etching process as to all the chips of the wafer, it is necessary for an etched-away depth to reach a level of 4500 angstroms corresponding to 1.5 times the film thickness of the actual polysilicon film. Consequently, when the selective ratio of the polysilicon film to the oxide film is equal to a ratio of 10 to 1, the gate insulation film 26 is over-etched away by the amount corresponding to a film thickness of 450 angstroms. Due to this, the total amount in film thickness of the gate insulation film 26 to be removed by etching reaches (300+450=) 750 angstroms. Therefore, after completion of the etching process, the gate insulation film 26 has a film thickness of (800xe2x88x92750=) 50 angstroms. In general, the etched-away amount in the wafer area varies within a range of approximately 10%. For example, as for the gate insulation film 26, its etched-away amount varies in depth within a range of approximately 75 angstroms. Due to this, the gate insulation film 26 is often completely removed in some chips of the wafer. As a result, as is clear from FIG. 44(b), a surface of the photoelectric conversion portion 29 is often damaged by etching. The thus damaged portion is denoted by the reference numeral 38 in FIG. 44(b). Due to such damaged portion 38, dark current increases in the photoelectric conversion portion 29 and crystal defects occur, whereby a so-called xe2x80x9cwhite damagexe2x80x9d occurs. Such deficiencies not only impair in properties the solid-state imaging device, but also considerably decrease the yield of the solid-state imaging device, which increases the manufacturing cost of the device.
In order to prevent the damage caused by etching from occurring, the gate insulation film 26 is increased in film thickness (hereinafter referred to as the former case), or the charge transfer electrode 32 is decreased in film thickness (hereinafter referred to as the latter case). However, the former case presents a new problem of a decrease in maximum quantity limit of charge transferred in the charge transfer portion 25. On the other hand, the latter case presents another new problem of an increase in resistance of the charge transfer electrode 32, which deforms in pulse-wave shape the transferred pulse.
Further, in the manufacturing method of the solid-state imaging device described in the first and the second conventional examples, as shown in FIGS. 33(a) to 42(b), in order to form the charge transfer electrodes 10, 32 together with the photoelectric conversion portions 6, 29 and the p+ type regions 12, 35, it is necessary to carry out the photoresist process three times. However, such burdensome repetition of the photoresist processes increases both the manufacturing time and cost of the device.
It is an object of the present invention to provide a solid-state imaging device and manufacturing method thereof, wherein the solid-state imaging device is capable of: preventing its readout characteristics of the signal charge from varying; preventing the dark current from increasing; preventing the so-called xe2x80x9cwhite damagexe2x80x9d from occurring; and, decreasing the manufacturing time and cost of the device.
Means for solving the problems inherent in the related art are as follows:
According to a first aspect of the present invention, the above object of the present invention is accomplished by providing:
A solid-state imaging device comprising:
a plurality of photoelectric conversion portions each for converting incident light into signal charge the amount in charge of which corresponds to the amount of the incident light;
a plurality of charge readout portions provided adjacent to the photoelectric conversion portions, each of the charge readout portions being designed to read out the signal charge having been generated in the corresponding one of the photoelectric conversion portions;
a plurality of charge transfer portions provided adjacent to the photoelectric conversion portions, each of the charge transfer portions being designed to transfer the signal charge having been retrieved from the corresponding one of the photoelectric conversion portions through the corresponding one of the charge readout portions; and
a charge transfer electrode, which is formed, through an insulation film, over the corresponding one of the photoelectric conversion portions, the corresponding one of the charge readout portions, the corresponding one of the charge transfer portions and also over peripheral portions of these corresponding ones, is provided with an opening portion over the corresponding one of the photoelectric conversion portions, and doubles a charge readout electrode for controlling the reading and writing of the signal charge from the corresponding one of the photoelectric conversion portions to the corresponding one of the charge transfer portions, the charge transfer electrode being designed to control in transfer the signal charge of the corresponding one of the charge transfer portions.
In this first aspect of the present invention, preferably, each of the photoelectric conversion portions is formed within a surface region of a first conductivity type semiconductor layer, and formed of a second conductivity type semiconductor layer.
According to a second aspect of the present invention, the above object of the present invention is accomplished by providing:
A solid-state imaging device comprising:
a plurality of photoelectric conversion portions each for converting incident light into signal charge the amount in charge of which corresponds to the amount of the incident light, the photoelectric conversion portions being formed within a surface region of a first conductivity type semiconductor layer and formed of a second conductivity type semiconductor layer;
a plurality of charge readout portions provided adjacent to the photoelectric conversion portions, each of the charge readout portions being designed to read out the signal charge having been generated in the corresponding one of the photoelectric conversion portions, the charge readout portions being formed of the first conductivity type semiconductor layer;
a plurality of charge transfer portions adjacent to the photoelectric conversion portions, each of the charge transfer portions being designed to transfer the signal charge having been retrieved from the corresponding one of the photoelectric conversion portions through the corresponding one of the charge readout portions, the charge transfer portions being formed of the second conductivity semiconductor layer;
a plurality of first charge transfer electrodes forming at least one layer through an insulation film over the corresponding one of the charge transfer portions, each of the first charge transfer electrodes being designed to control in transfer the signal charge of the corresponding one of the charge transfer portions; and a plurality of second charge transfer electrodes, each of which is formed, through an insulation film, over the corresponding one of the photoelectric conversion portions, the corresponding one of the charge readout portions, the corresponding one of the charge transfer portions and also over peripheral portions of these corresponding portions, is provided with an opening portion over the corresponding one of the photoelectric conversion portions, wherein adjacent ones of the second charge transfer electrodes are formed so as to be separated from each other through a separation portion separated from the opening portion, and each of the second charge transfer electrodes doubles as a charge readout electrode for controlling the reading and writing of the signal charge from the corresponding one of the photoelectric conversion portions to the corresponding one of the charge transfer portions, the second charge transfer electrode being designed to control in transfer the signal charge of the corresponding one of the charge transfer portions.
In the second aspect of the present invention, preferably, each of the second charge transfer electrodes is formed in a manner such that it is spaced a predetermined distance away from any one of the corresponding ones of the first charge transfer electrodes in one and the same layer, or has its edge portion overlie a corresponding edge portion of the any one of the corresponding ones of the first charge transfer electrodes in different layers spaced apart from each other through an insulation film.
In the first and the second aspect of the present invention, preferably, each of the photoelectric conversion portions is formed by self alignment using an edge portion of an opening portion of the corresponding one of the charge transfer electrodes as a mask.
Further, preferably, each of the photoelectric conversion portions is constructed of: a first one of the second conductivity type semiconductor layer, the first one being large in depth and narrow in its region; a second one of the second conductivity type semiconductor layer, the second one being small in depth and wide in its region; and, the first conductivity type semiconductor layer, the first conductivity type semiconductor layer being formed in surface regions of these first and second ones of the second conductivity type semiconductor layer.
Still further, preferably, the first conductivity type semiconductor region forming the each of the photoelectric conversion portions is formed in a manner such that the first conductivity type semiconductor region is spaced a predetermined distance away from an edge portion of the charge readout portion side of the opening portion of the corresponding one of the charge transfer electrodes.
According to a third aspect of the present invention, the above object of the present invention is accomplished by providing:
A manufacturing method of a solid-state imaging device, the solid-state imaging device comprising: a plurality of photoelectric conversion portions each for converting incident light into signal charge the amount in charge of which corresponds to the amount of the incident light; a plurality of charge readout portions provided adjacent to the photoelectric conversion portions, each of the charge readout portions being designed to read out the signal charge having been generated in the corresponding one of the photoelectric conversion portions; a plurality of charge transfer portions provided adjacent to the photoelectric conversion portions, each of the charge transfer portions being designed to transfer the signal charge having been retrieved from the corresponding one of the photoelectric conversion portions through the corresponding one of the charge readout portions; and, a charge transfer electrode, which is formed, through an insulation film, over the corresponding one of the photoelectric conversion portions, the corresponding one of the charge readout portions, the corresponding one of the charge transfer portions and also over peripheral portions of these corresponding ones, the charge transfer electrode being designed to control in transfer the signal charge of the corresponding one of the charge transfer portions; the method comprising:
a first step of forming a plurality of first charge transfer electrodes each constructed of at least one layer, the first charge transfer electrodes being formed over a first conductivity type semiconductor layer, in a surface region of which are formed: the plurality of charge readout portions constructed of the first conductivity type semiconductor layer; and, the plurality of charge transfer portions constructed of the second conductivity type semiconductor layer;
a second step of forming a photoresist film having an opening portion in a region corresponding to a region in which the plurality of photoelectric conversion portions are formed after a conductive film is formed through an insulation film;
a third step of removing the conductive film of the region corresponding to the opening portion by using the photoresist film as a mask;
a fourth step of forming the plurality of photoelectric conversion portions by using both the photoresist film and the conductive film as masks, or by using the conductive film only as a mask; and
a fifth step of forming, by etching the conductive film, a plurality of second charge transfer electrodes each doubling as a charge readout electrode for controlling the reading and writing a signal charge from the corresponding one of the photoelectric conversion portions to the corresponding one of the charge transfer portions.
In this third aspect of the present invention, preferably, in the fifth step, adjacent ones of the second charge transfer electrodes are formed so as to be separated from each other through a separation portion separated from the opening portion.
Further, preferably, in the fifth step, each of the second charge transfer electrodes is formed in a manner such that it is spaced a predetermined distance away from any one of the corresponding ones of the first charge transfer electrodes in one and the same layer, or has its edge portion overlie a corresponding edge portion of the any one of the corresponding ones of the first charge transfer electrodes in different layers spaced apart from each other through an insulation film.
Still further, preferably, in the fourth step, after the plurality of second conductivity type semiconductor layers are formed by using both the photoresist film and the conductive film as masks, the first conductivity semiconductor region is formed in the surface region of each of the second conductivity type semiconductor layers by using both the photoresist film and the conductive film as masks, or, by using the conductive film only as a mask.
Further, preferably, in the fourth step, after the first ones of the second conductivity type semiconductor regions each of which is large in depth and narrow in region are formed by using as a mask a photoresist film provided with a second opening portion smaller in area size than the first opening portion, the second ones of the second conductivity type semiconductor regions each of which is small in depth and wide in region are formed by using as a mask the conductive film only, and, the first conductivity type semiconductor regions are formed in the surface regions of both the first and the second ones of the second conductivity type semiconductor regions.
Further, preferably, in the fourth step, the first conductivity type semiconductor regions are formed by ion-implanting a fist conductivity type impurity at a predetermined inclination implantation angle relative to a vertical direction in a manner such that each of the first conductivity type semiconductor regions is spaced a predetermined distance away from an edge portion of the charge readout portion side of the opening portion of the corresponding one of the charge transfer electrodes.
The effects of the present invention are as follows. Namely, as described above, in the present invention having the above construction, it is possible to suppress variations in the readout characteristics of the signal charge. Further, it is possible for the present invention to prevent the dark current from increasing, and also possible to prevent the so-called xe2x80x9cwhite damagexe2x80x9d from occurring. Furthermore, it is also possible to decrease the manufacturing time and cost of the solid-state imaging device.
In addition to the above, in the present invention having another construction, since the charge transfer electrode is composed of a single layer, it is possible to reduce the interlayer capacity, which may solve the problem of insulation between the electrodes.
In the present invention having further another construction, since the photoelectric conversion portion is composed of the first conductivity type semiconductor region and the second conductivity type semiconductor region, it is possible to align the edge portion of the charge transfer electrode with the edge portion of the photoelectric conversion portion without fail even when the edge portion of the charge transfer electrode doubling as a charge readout electrode is set back relative to the edge portion of the photoresist film due to over etching or due to variations in etching, which assures that a readout voltage of the signal charge further stabilized is obtained.
Further, in the present invention having still further another construction, since the first conductivity type region forming the photoelectric conversion portion is formed so as to be spaced apart from the edge portion of the charge transfer electrode (which doubles as a charge readout electrode) by a predetermined distance, there is no fear that the first conductivity type impurity in the first conductivity type region diffuses horizontally into a region located under the channel of the charge readout portion. Due to this, it is possible to reduce a readout-voltage of the signal charge.